1) Field of the Disclosure
The disclosure relates generally to reinforcement or toughening materials for composite structures, and more particularly, to nano-reinforcement filler materials for epoxy resin systems for composite structures, such as aircraft composite structures and other composite structures.
2) Description of Related Art
Composite materials are used in a wide variety of structures and component parts, including in the manufacture of aircraft, spacecraft, rotorcraft, watercraft, automobiles, trucks, and other vehicles, because of their high strength-to-weight ratios, corrosion resistance, and other favorable properties. In particular, in aircraft construction, composite structures and component parts are used in increasing quantities to form the fuselage, wings, tail section, skin panels, and other component parts of the aircraft.
Epoxy resins are commonly used as matrices for fiber-reinforced composite materials due to their high temperature performance, chemical resistance, and good adhesive properties. Such epoxy resins include epoxy polymers which are thermosetting structural polymers that become permanently hard after curing. However, epoxy polymers have a three-dimensional crosslinked structure that may result in a low resistance to crack propagation. Thus, reinforcement or toughening filler materials may need to be added to epoxy resins to improve fracture toughness, as well as to improve other mechanical and other physical properties.
Known reinforcement or toughening filler materials for epoxy resins exist. For example, such known reinforcement or toughening filler materials may include carboxyl-terminated butadiene-acrylonitrile (CTBN) rubbers and core-shell glass beads-polymer microspheres. However, while such known reinforcement or toughening filler materials may improve fracture toughness, they may adversely affect strength and other physical properties such as thermal properties.
In addition, known reinforcement or toughening filler materials for epoxy resins include nano-reinforcement filler materials such as inorganic silica nanoparticles. However, such inorganic silica nanoparticles have a high density which may result in undesirable high weight composites. High weight composite structures may be undesirable for aircraft, spacecraft, and other vehicles, as increased weight results in increased use of fuel, and in turn, increased costs. Thus, a composite material that enables the manufacture of lower weight structures is advantageous and desirable.
Further, known nano-reinforcement filler materials for epoxy resins include graphene and modified graphene reinforcement filler materials, such as graphene oxide (GO) nanosheets, i.e., GO sheets, obtained by oxidative exfoliation of graphite. Graphene oxide (GO) sheets have a large number of hydroxyl, epoxy, and carboxyl groups, which are chemically compatible with epoxy resin, at its surfaces and edges, and which provide the GO sheets with good functionalization, processing, and water solubility. Investigation of graphene oxide (GO) sheets has been made to enhance mechanical properties without compromising other physical properties at extremely low loading levels of 0.1 to 3 wt % (weight percent). However, there may be various issues, such as agglomeration of particles. In addition, it is not fully understood as to how the shape and size of such GO sheets may be controlled, which may influence the resulting properties of the composites.
In addition, when such known graphene oxide (GO) sheets (nanosheets) undergo a drying process after synthesis, they may undesirably restack on themselves. Use of a solvent may be needed to prevent or avoid the restacking of the individual GO sheets. However, the presence of any residual solvent may interfere with the crosslinking reaction between the GO sheets and the epoxy resin matrix and may adversely affect the mechanical properties of the epoxy resin matrix. To prevent or avoid the presence of any residual solvent, solvent removal may be required. Such solvent removal may involve the use of a vacuum oven or another solvent removal apparatus to remove the solvent. Such solvent removal may be very time consuming, i.e., 23 h (twenty-three hours), or more, and may be very costly. Although solvent-free loading has been used for synthesis of GO wrapped inorganic silica nanoparticles, there is a need for solvent-free loading for synthesis of GO wrapped organic nanoparticles, such as polymer nanoparticles.
Further, known nano-reinforcement filler materials for epoxy resins include known graphene oxide (GO) sheets (nanosheets) used with nanoparticles, such as polymer nanoparticles. However, such known GO wrapped polymer nanoparticles include polymer nanoparticles ranging in size from 350 nm (nanometers) to a few microns, which may, in turn, require the use of large size GO sheets. Large polymer nanoparticles may result in a reduction in the surface area of interaction or interface between the GO sheets and the epoxy resin matrix.
Moreover, large size GO sheets may cover many particles with a single GO sheet, resulting in agglomerated particle structure. Agglomerated particles wrapped in a single GO sheet may interfere in the curing reaction of epoxy with the resin matrix, which may, in turn, result in unsatisfactory mechanical properties of the nano-reinforcement filler materials.
Accordingly, there is a need in the art for an improved nano-reinforcement filler material and methods for making the same, used with epoxy resin systems for composite structures, that provide an enhanced interaction or with the epoxy resin matrix, that are solvent-free, that reduce synthesis or fabrication time, that reduce weight of the composite structure, and that provide advantages over known nano-reinforcement filler material materials and methods.